Humanitarian agencies have pulled out of Pagak, the headquarters of the SPLM-In Opposition in South Sudan's Upper Nile state, due to harassment and threats.

The UN's humanitarian coordinator for South Sudan Toby Lanzer said conditions for aid agencies in the town have deteriorated lately.

"Aid workers there have been harassed and threatened and their freedom of movement has been impeded," Lanzer said in a statement. "The organisations are no longer able to conduct their work on behalf of populations in need in an impartial, neutral, and safe manner. As a consequence, aid workers have left Pagak until such a time when the conditions for them to be present there, on behalf of civilians in need, are re-established."

Lanzer said the safety and security of aid workers and their property is a condition for emergency relief services to to be carried out, noting that both South Sudanese and foreign humanitarian staff members must be free to move unimpeded to reach civilians in need. He said humanitarians work in accordance with principles of "humanity, impartiality, and neutrality."

The announcement of harassment and threats toward humanitarian workers comes as the SPLM-In Opposition rebel movement led by Riek Machar has held a consultative conference in Pagak.

File photo: military leaders at an opposition conference in Pagak in December 2014